Miniaturization of chemical analysis is a highly active area of intense scientific research. Much of the research is driven by the health and life sciences, where miniaturization has the capacity to revolutionize the diagnosis and treatment of disease [Yager et. Al Nature 2006, 442, 412-418; Chin, Linder, Sia Lab Chip, 2007, 7, 41-57]. Central to this theme is the miniaturization of processes and procedures that occur in conventional chemical and biological laboratories. These activities include sampling, storage, sample treatment, separation, detection, and analysis. Miniaturization uses less sample, offers superior detection sensitivity, and has the potential to greatly reduce the costs of laboratory environment, labor, and materials. Efforts at miniaturization have focused primarily on the implementation of so-called microfluidic “lab-on-chip” devices [Chin, Linder, Sia, Lab Chip, 2007, 7, 41-57], although more conventional methods, such as lateral flow chromatography, have also been reduced in scale [Yager et. Al Nature 2006, 442, 412-418].
A particularly promising analytical technology for medical diagnostics from biological tissues and fluids is liquid chromatography coupled to mass spectrometry (LC-MS) [Hoofnagle, Clin. Chem. 2010, 56, 161-164; Anderson Clin. Chem. 2010, 56, 177-185]. LC-MS is a powerful method, but requires a highly complex analytical system. Current state-of-the-art practice requires expert level training of staff, together with a significant investment in laboratory infrastructure. Centralized laboratory resources coupled together with remote sampling of patient populations is a common solution to meet these multiple requirements.
Electrospray ionization is a well-established method to ionize liquid samples for chemical analysis by mass spectrometry. Nanoelectrospray ionization, also referred to as nanospray, is a miniaturized low-flow and low-volume variant of electrospray ionization. Nanospray has been shown to offer superior sensitivity and selectivity compared to conventional electrospray ionization. Various methods exist in the prior art for using nanospray for the on-line analysis of flowing liquid streams, e.g. the effluent from liquid chromatography.
A commonly employed apparatus for on-line nanospray utilizes a nanospray emitter fabricated from a tube, typically 50 to 300 μm inside diameter (ID), having a finely tapered end in which the ID tapers to a 1-20 μm ID orifice. The tapered end is referred to as the proximal end. A high voltage (1-4 kV) is applied to the liquid mobile phase resulting in an electrically charged aerosol emitting from the proximal end during the electrospray process. Some portion of the generated charged aerosol is collected by the inlet orifice of the mass spectrometer for chemical analysis.
Emitters are generally fabricated from tubing made from borosilicate glass, fused-silica, or fused quartz, although other materials including polymers and metals have been employed. The non-tapered end is referred to as the distal end, and is the end of the emitter through with sample and mobile phase enter the emitter. Suitable emitters may contain a sorbent material within the inner bore of the tube for use as a chromatography column for separations or analyte capture and purification, such as that described by U.S. Pat. No. 5,572,023 to Caprioli.
A significant challenge for successful on-line nanospray is multi-fold. These methods are typically time consuming, expensive, and/or require a great deal of hand manipulation and fine motor skills. The nanospray emitters are fairly delicate and fragile. The small ID's for the emitters (<20 μm) require the use of specialized tools. Expert level training is usually required for successful application of the technique. Making fluidic connections that are both leak-free and capable of withstanding high internal operating pressure with tubing that is on the order of 100 μm (0.004″)ID and smaller requires a significant investment in operator training. Improper assembly often results in either clogging of tubing or leaks that often go undetected.
Thus there is a significant need for a miniaturized system providing high chromatographic performance and high analytical sensitivity combined with robustness and ease of use for and analysis of samples by liquid chromatography and nanospray ionization mass spectrometry. It is particularly desirable that the system be easy-to-use, be low cost, and offer high throughput. It should be usable with a minimum of specialized laboratory equipment, preferably require only those tools commonly found in a clinical laboratory or hospital environment.